Maintained by Robin Tecon, microbiologist and postdoctoral researcher at the Swiss Federal Institute of Technology Zürich. This blog is about bacteria (and other microbes) and the scientists who study them.

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Tuesday, November 27, 2012

The comeback of whooping cough

I read a very interesting—and somewhat alarming—article in
the October issue of the journal Microbe (formerly ASM News). In this article,
Merry Buckley explains that whooping cough (aka pertussis), a childhood disease
that has strongly declined since the introduction of a vaccine in the 1940s, is
now on the rise again. We even see epidemics! says Buckley.

In 2012, in the
USA, the number of pertussis cases is expected to be the highest in fifty
years, approaching 40,000. Other countries such as Australia and the
Netherlands also have high incidence of the disease—in the Netherlands, 6,000
cases were reported in 2009, against only 30 cases in 1980. The causes of this
comeback are not fully understood, but scientists have gathered many clues.

Whooping cough is caused by Bordetella pertussis, a Gram negative bacterium belonging to the
group beta Proteobacteria. It infects the respiratory system, causing a
characteristic ‘whoop’ sound in sick children. Teens and adults can also be
infected, although the symptoms are milder than in small children. In the
prevaccine era, pertussis was a terrible threat, killing on average 5,000 children
a year during the 1920s and 30s in the USA alone. At this period, the epidemics
peaked following a cyclic pattern of a couple of years.

When the vaccine, which
consisted of killed B. pertussis
bacteria, became in use (together with the diphteria and tetanus vaccines), the
number of pertussis cases sharply dropped down. During the 1980s, however, the
trend was reversed and more cases were reported in the US, this despite the
fact that 95% of the children were vaccinated. In 1991 a new pertussis vaccine,
which was based on antigens only, was implemented, and it is this one that is
in use today. But the introduction of a new vaccine did not stop the climb
initiated in the 1980s, and since 2000 we have seen again the cyclic
pattern of epidemics peaking every five years or so. The only positive note is
that, despite the increase in the disease incidence, the number of fatal cases remains
low.

So why is pertussis on the rise? One reason is waning immunity. More adult and teens become
infected and, even though they don’t risk much for themselves, these carriers
become a source of infection for children. This decrease in the vaccine
protection overtime may possibly be linked to the use of the acellular vaccine.
But it is not sufficient to explain the present situation: genetic changes in
the populations of Bordetella pertussis
likely are part of the story. In particular, researchers point at variations in
the promoter of the gene encoding the pertussis toxin, and at the virulence
factor pertactin. New variants that express more toxic (via changes in the
promoter) and that have a pertactin that is not recognized by the immune system
seemingly contribute to the rise of pertussis in Europe and Australia. The
situation is somewhat different in the US, where a different trait—the
production of a certain type of fimbriae—is associated with the rise of the
disease. In both cases, however, it is a serious possibility that vaccination
has helped select for these more dangerous variants.

What can be done to counter this situation? Well, the
clinicians working on pertussis suggest that the current vaccine should be
carefully reviewed and, possibly, replaced by a more up-to-date version.